In recent years, in the medical and biotechnology fields, circulating tumor cells which are cancer cells invading blood from cancer tissue have attracted attention (refer to Non-Patent Document 1). That is, as one of the cancer metastasis mechanisms, there is hematogenous dissemination in which cancer cells developed in a certain organ are transported by blood and metastasize to other organs. It has been expected that metastasis of cancer can be suppressed by removing cancer cells in blood. It is hoped that if living cancer cells can be captured from blood, this will contribute to the development of medicines as antibodies, and the properties and mechanisms of metastasis of cancer will be unraveled.
Blood cells in blood are briefly classified into red blood cells and white blood cells. Approximately 5×109 red blood cells are contained in 1 mL of whole blood. On the other hand, approximately 5×106 to 1×107 white blood cells are contained in 1 mL of whole blood. That is, it is said that the concentration of white blood cells is 1/500 to 1/1000 of that of the red blood cells. A red blood cell has a disk shape, a diameter of 7 to 8.8 μm, and a thickness of 2 to 3 μm. White blood cells are classified into monocytes, granulocytes, and lymphocytes. The size of a monocyte is 13 to 21 μm, the size of a granulocyte is 10 to 18 μm, and the size of a lymphocyte is 7 to 16 μm. The abundance ratio of monocytes, granulocytes, and lymphocytes is 7:57:36.
On the other hand, cancer cells which break the basement membrane and mix with blood from cancer tissue have a size of approximately 20 μm, and their nuclei are hypertrophied. The ratio of cancer cells to nucleated cells (white blood cells) in blood is 1/107, and the number of cancer cells per 1 mL of blood is 1 (refer to Non-Patent Documents 2 and 3). Approximately 90% of cancer cells can be discriminated based on only their sizes from blood cells (refer to Non-Patent Document 4).
As a technique for separating cancer cells from blood, a method using a physical mesh by utilizing the size difference between blood cells and cancer cells is known (refer to Non-Patent Documents 5 and 6). However, this physical mesh is easily clogged.
Generally, it is said that the blood volume of a human is 7 to 8% of his/her body weight, and the blood volume of an adult is estimated to be approximately 5000 mL. When the total amount of blood is examined, the examinee is confined during examination, so that the throughput of the blood examination apparatus is a very important factor. The most standard apparatus for examining cells is flow cytometry.
The processing ability of this flow cytometry is 100,000 blood cells per second at most. Focusing on only the number of white blood cells, approximately 107 white blood cells are contained in 1 mL of blood, so that when 5000 mL of blood is examined by flow cytometry, the examination takes 150 hours. In actuality, it has been attempted to detect cancer cells in blood by using flow cytometry (refer to Non-Patent Documents 7 to 9), however, the flow cytometry is not an apparatus intended to examine as much blood as 5000 mL.
In the flow cytometry, cells are made to flow one by one, and fluorescence and scattered light from individual cells are received by a photodetector, and the waveform of an electric signal output from the photodetector which receives light is analyzed to identify the cells. The throughput of the flow cytometry is rate-controlled to the time (dead time) necessary for such analysis of the waveform of the electric signal (refer to Non-Patent Document 10). Thus, according to the flow cytometry, living cells cannot be identified at a frequency of 1/107 (1 per 1 mL of blood) from 5000 mL of blood.
It is possible that blood is imaged by using a CCD camera or a CMOS camera and the image obtained through the imaging is analyzed to examine whether a cancer cell is present in the image. CCD cameras and CMOS cameras have been increased in speed according to improvement in imaging techniques, however, the frame rate is 5 kHz at most, and the image update time interval is 200 μsec. On the other hand, the migration speed of cells in the flow cytometry is several meters per second. For example, in a microscopic objective lens field of 40 times (approximately 0.5 mm square), a cell moving at a speed of 1 m/sec passes through the field (comes out of the frame) in 500 μsec. Therefore, it is not realistic to find cancer cells in a large amount of blood by analyzing images taken by a CCD camera or a CMOS camera.
A technique for identifying cancer cells in blood by using a matched filter (holographic filter) method (refer to Patent Document 1) is known. According to this technique, a pattern of diffracted light generated from blood irradiated with a laser beam is formed, and an image of light output from a matched filter disposed on the diffracted light pattern formed surface is imaged by a CCD camera, and by analyzing the image obtained through this imaging, the position, etc., of a cancer cell in blood is detected. However, this technique also requires analysis of the images taken by the CCD camera, and therefore, it is not realistic to find cancer cells in a large amount of blood. In Patent Document 1, a detailed matched filter shape for finding cancer cells in blood is not described, and the problem is unsolved.    Patent Document 1: Japanese Patent Registration No. 2582797    Non-Patent Document 1: M. Cristofanilli, et al., The New England Journal of Medicine, Vol. 351, pp. 781-791, (2004).    Non-Patent Document 2: L. W. M. M. Terstappen, et al., International Journal of Oncology, Vol. 17, pp. 573-578, (2000).    Non-Patent Document 3: H. B. Hsieh, et al., Biosensors and Bioelectronics, Vol. 21, pp. 1893-1899, (2006).    Non-Patent Document 4: L. A. Liotta, et al., Cancer Research, Vol. 34, pp. 997-1004, (1974).    Non-Patent Document 5: G. Vona, et al., American Journal of Pathology, Vol. 156, No. 1, pp. 57-63, (2000).    Non-Patent Document 6: P. Rostagno, et al., Anticancer Research, Vol. 17, pp. 2481-2485, (1997).    Non-Patent Document 7: A. L. Allan, et al., Cytometry Part A, Vol. 65A, pp. 4-14, (2005).    Non-Patent Document 8: H-J. Gross, et al., Cytometry, Vol. 14, pp. 519-526, (1993).    Non-Patent Document 9: H-J. Gross, et al., Proc. Natl. Acad. Sci. USA, Vol. 92, pp. 537-541, (1995).    Non-Patent Document 10: J. F. Leary, Methods in Cell Biology, Vol. 42, Chapter 20, pp. 331-358, (1994).